In recent years, applications of filler-containing polypropylene resins to automobile parts and appliances have been markedly broadened because of their excellent mechanical and thermal properties.
In the application to automobile parts, for instance, interior parts, such as an instrument board, a trim, and a pillar-post, are often covered with non-woven fabric, foamed fabric, leather cloth made of polyvinyl chloride, and the like to produce an atmosphere of high-class. Further, there is noted a tendency to coat a bumper in color harmony with the body in a decorative style. In appliance parts, also, printing is commonly employed for decoration.
Polypropylene resins often find difficulty in finishing due to their poor adhesion or coating properties arising from non-polarity. It is well known to add fillers to polypropylene resins in order to improve these finishing properties. Incorporation of fillers improves finishing properties to some extent depending on the kind and amount of the filler, but such an improvement is still insufficient for practical use.
In order to further improve finishing properties, i.e., compatibility between polypropylene and adhesives or coatings, a medium called "primer", which typically comprises chlorinated polypropylene and toluene, etc., is usually provided therebetween. However, since the primer itself is expensive and an extra coating step is required, the cost of the final products is so increased. Therefore, further improvements have been demanded.
In an attempt to improve surface properties, physical or chemical etching of the surface of polypropylene molded products prior to adhesion has been studies and applied to practice. However, this process involves more complicated steps than the primer coating, and some conditions of etching cause deformation of molded products or produce insufficient effects.
In the case of coating, the process of coating a primer is being shifted to a process comprising oxidatively polarizing the surface of molded articles by irradiation with plasma to improve coating properties. Under the present situation, however, this process does not always achieve satisfactory results because the plasma irradiation should be effected in a batch system and the polarity obtained is non-uniform depending on the shape of the molded articles.
Attempts have also been made to impart polarity to polypropylene per se. For example, it has been proposed to modify polypropylene by grafting with an unsaturated carboxylic acid or an anhydride thereof, e.g., acrylic acid, maleic anhydride, etc., in the presence of an initiator, e.g., organic peroxides, or to blend the above-described modified polypropylene with unmodified polypropylene, as disclosed in Japanese Patent Application (OPI) No. 76149/75 (the term "OPI" as used herein means "unexamined published application"), Japanese Patent Publication No. 10264/77, etc.
According to these processes, the grafting is accompanied by decomposition of polypropylene with the peroxide so that the amount of the unsaturated carboxylic acid or the anhydride thereof to be incorporated is limited. As a result, the surface modifying effect attained is insufficient. If any effects are produced, mechanical properties are impaired due to molecular weight reduction of polypropylene.
In order to overcome these disadvantages associated with the conventional polypropylene resins, the present inventors proposed a composition comprising a polypropylene resin, an ethylene copolymer composed of an ethylene unit, an ester unit selected from alkyl acrylates and alkyl methacrylates, and an unsaturated dicarboxylic acid anhydride unit, and a filler, as disclosed in European Patent No. 213,720A.
However, the aforesaid filler-containing polypropylene resin composition is insufficient in manifestation of the effects of fillers to improve rigidity and also exhibits unsatisfactory thermal properties, although surface modifying effects can be achieved as expected.